As the size of semiconductor devices has decreased, photolithographic techniques become unable to reliably create structures of the dimensions required. As photolithographic techniques have become unusable, other technologies have been developed to create the small structures required by the ever shrinking semiconductor devices. One example of a non-photolithographic imaging technique is sidewall image transfer (“SIT”).
SIT is able to produce structures substantially narrower than the minimum size achievable with photolithographic techniques, while maintaining excellent width control. However, SIT methods produce structures, usually hard masks, of generally closed-loop geometry. These loops have a single, well-controlled width (referred to henceforth as the “critical image width”). Conventional SIT applications thus require the use of two additional masks. One, called a “loop cutter mask”, is employed to segment the loops, and a second, called a wide-area mask, is employed to add shapes of any other, usually wider, dimensions.
Accordingly, subsequent processing is required to remove the unwanted portions of the closed-loop structures produced by SIT. While removing unwanted loop material with the loop-cutter mask, both edges of loop material which define portions of critical width images must be protected in order to maintain the excellent image size control afforded by SIT techniques. Both of these edges must subsequently be left exposed during etch of underlying films in order to accurately transfer their pattern into those underlying films.
Furthermore, not all structures being formed are to be of the same small dimensions typically produced by SIT methods. Wider shapes are often required, for example, to fabricate contact landing pads.
Combining other imaging processes with SIT methods to produce structures of varying dimensions, with some of those dimensions smaller than those achievable by photolithographic techniques, is typically required.